Method of making pressure vessels



March 15, 1966 F. WOLFF 3,240,644

METHOD OF MAKING PRESSURE VESSELS Filed Nov. 2, 1962 INVENTOR D FRANK WOLFF BY AT RNEY United States Patent C) 3,240,644 METHOD OF MAKING PRESSURE VESSELS Frank Wollf, River Edge, N.J., assignor to Specialties Development Corporation, Belleville, N.J., a corporation of New Jersey Filed Nov. 2, 1962, Ser. No. 234,923- 3 Claims. (Cl. 156-165) The present invention relates to vessels for storing fluid under pressure, and, more particularly, to an improved method of making such vessels.

Pressure vessels are used extensively for storing fluid adapted to be utilized to provide auxiliary power or for other purposes on aircrafts, rockets, missiles and other airborne devices. In such applications of use, it is desirable to store a maximum of pressure fluid in a vessel occupying a minimum of space which requires that the fluid be stored under high pressure. The trend has been to store fluids at higher and higher pressures over the years wherefore today it is not unusual to store the fluid at pressures as high as 3,500 p.s.i. or even higher. In view of the fact that weight is a penalty in all airborne applications, there has been a constant struggle to produce vessels having maximum strength and minimum weight for a particular high service pressure.

In the past, pressure vessels have been fabricated which comprised an inner steel shell and a winding of piano wire on the shell. This winding permitted the use of a somewhat thinner and lighter shell to produce a vessel of slightly less weight for 'a given service pressure. The primary advantage of such vessels has been that the wire winding prevents shattering of the steel shell when struck by gunfire.

Also, pressure vessels have been fabricated which comprised a thin gas tight inner shell or liner and an outer shell of considerable strength and wall thickness composed of a resin impregnated fiberglass winding. Such vessels are relatively light in weight and are non-shatterable. However, such vessels cannot be tested without destruction thereof to locate flaws and imperfections which would render the vessels unsafe in service.

More recently, it has been proposed to improve the structural strength of steel shells by pre-stressing the same. Such prestressing has been attempted either by Winding high tensile strength filaments on steel shells or by internally pressurizing beyond the yield point steel shells having a non-tensioned winding thereon and then releasing the pressure. However, neither method has been satis factory. The winding tension method is limited to relatively low values of prestress in the winding; and the yield pressure method is limited by the ductility of the steel because a high degree of strain hardening would result which would considerably lower the ductility of the steel.

In accordance with the present invention, it has been discovered that cetrain features of the winding tension and yield pressure methods can be combined and utilized under controlled conditions to produce safe and uniform vessels having a high strength for weight ratio and having a greater cycle life than those produced by either of these pre-stressing methods alone or by any of the other methods heretofore employed in producing nonwound metallic pressure vessels, the vessels being of comparable strength to weight.

Accordingly, an object of the present invention is to 'ice provide a method of making high strength, low weight pressure vessels in a simple, practical and economical manner.

Another object is to provide a method of producing such vessels which is particularly adapted for use in connection with vessels of generally cylindrical shape.

A further object is to provide such a method which is adapted for use in connection with a wide variety of high strength, ductile metals or metallic alloys and high tensile strength continuous filaments.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

In the drawing, the single figure is a fragmentary elevational view, partly in section, of a cylindrical pressure vessel in accordance with the present invention.

Referring now to the drawing in details, there is shown a pressure vessel which comprises a shell 10 formed of high strength, ductile metal having a cylindrical side wall 11 and having hemispherical end walls 12 one of which is provided with an opening 14 for introducing and discharging fluid under pressure; and a hoop Winding 15 of a plurality of layers of resin coated high tensile strength continuous filaments wound under tension on a section of the cylindrical wall.

The method utilized to produce the pressure vessel genearlly comprises applying the hoop winding 15 at a tension to partially pre-stress the cylindrical wall section, curing the resin to provide a permanent filamentresin laminate on the wall 11, and subjecting the interior of the shell 10 to fluid pressure at a value and for a duration to expand the shell and cause the yield strength to approach the tensile strength of the metal providing the cylindrical section to further pre-stress the cylindrical wall.

More specifically, the shell 10 is constructed of steel and has a substantially uniform wall thickness, and the winding is composed of fiberglass and epoxy resin. The layers of the winding are applied with decreasing tension from the first to the last layer. The weight of the cylindrical wall section covered by the winding is between about three and about five times the weight of the laminate on the wall section. The fluid pressure to which the interior of the shell is subjected is at a value between about halfway between the test pressure and burst pressure and a value closely approaching the burst pressure. This pressure is applied to the shell for a duration of between about one minute and about two minutes.

As a specific example of practicing present invention, eight nominal 520 cubic inch cylindrical shells, Nos. 1, 2, 3, 4, 5, 6, 7 and 8 were constructed of SAE 4130 steel (a ductile, high strength chrom-moly steel which maintains its properties 65 F. and +l60 F.), and were heat treated to provide the same with 152,500 p.s.i. ultimate tensile strength. These shells had a substantially uniform wall thickness of about 0.100 inch, an overall length L of about 19.12 inches, a diameter D of about 6.78 inches, and a cylindrical wall section CS to be covered by the winding 15 having a length of about 12.2 inches. These shells had -a weight of about 12.9 pounds with the cylindrical wall section having a Weight of about 7.3 pounds.

Shell No. 1 was provided with a winding composed of twenty-seven ends of fiber glass wound in seven layers of about 141 turns per layer laid down neatly without gaps. The winding is applied with a decreasing tension schedule. For example, the first layer may be applied under twentyfive to twenty-seven pounds of tension and the seventh or last layer may be applied under nineteen to twentytwo pounds of tension. After the first or second layer, the tension may be reduced for each layer or each other layer. The following tension schedule was employed:

Pounds First layer 27 Second layer 27 Third layer 23 Fourth layer 23 Fifth layer 19 Sixth layer l9 Seventh layer 19 The fiberglass was coated with suflicient epoxy resin so that the cured laminate had a content of about 85% by weight of glass and about 15% by weight of resin, and the laminate had a weight of about 1.2 pound (weight of steel to laminate at CS, about 6 to 1) and a thickness of 0.067 inch. The laminate was cured for two hours at 185 F. and then for six hours at 250 F.

Such a laminate has an ultimate tensile strength of about 220,000 p.s.i. The fiberglass by being tensioned during the winding thereof in the foregoing described manner created about 22,100 p.s.i. tensile stress in the laminate which is eflective to pre-strain the cylindrical wall section CS of the shell about 0.005 inch per inch to provide a compressive pre-stress therein of about 14,800 p.s.i.

After the laminate was applied and cured, a fluid pressure of about 6,900 p.s.i. was produced by hydraulic fluid and was maintained in the interior of the shell for about one minute and then was released.

The remaining permanent set in the steel was found to be 5,235 micro inches per inch which imparted a tensile pre-stress of 38,200 p.s.i. in the laminate and this pre-stress in turn imparted a compressive pre-stress of 25,600 p.s.i. in the steel. The total compressive prestress by the use of combined winding tension and yield pressure methods in the foregoing shell was about 40,400 p.s.i. The combined tensile pre-stress in the laminate was 60,300 p.s.i. In an instrumented burst test, the 0.2% offset yield pressure of this vessel occurred at 7,400 p.s.i., and the vessel burst at 8,460.

The customary hydrostatic proof test to which pressure vessels are subjected was not required because the shell was already subjected to a pressure of 6,900 p.s.i. for one minute during the pre-stressing thereof whereby the observations made at that time provided the necessary proof test data.

Vessels made in accordance with the foregoing procedure fully met the following operating requirements:

Service pressure 3,600 p.s.i.

Test pressure 6,000 p.s.i.

Burst pressure (minimum) 8,000 p.s.i.

Operating temperature 65 F. to 160 F.

Cycle life (minimum) 10,000 cycles from to 3,600 p.s.i.

Shell No. 2 was provided with a winding 15 which was applied at a very low tension, and was subjected to internal fluid pressure like Shell No. 1.

Shell No. 3 was provided with a winding 15 identical to that of Shell No. 1, but was not subjected to any internal fluid pressure.

Shell No. 4 was provided with a winding 15 identical to that of Shell No. 2, but was not subjected to any internal fluid pressure.

Shell No. 5 was not provided with a winding to pre-stress the same, but was pre-strained to approximately the same permanent set as Shell No. 1 by subjecting it to an internal fluid pressure of about 4,600 p.s.i. for one minute and then releasing the same.

Shell No. 5 was not provided with a winding 15 to subjected to any internal fluid pressure.

The effects of winding and/or yield pressure prestressing and the lack of either were investigated by means of strain gauge instrumented tests. The results of these tests are given in the following table:

TABLE I Winding Yield Pre-strain, Total 0.2% oil- Shell pre-stress, pressure ln/lnXlO- prestress, set yield N o. p.s.i. pre-stress, p.s.i. pressure p.s.i.

In addition, a longitudinal tensile specimen was cut from Shell No. 7 which was practically identical to Shell No. 1 prior to the burst test, and a similar specimen was cut from Shell N0. 8 which was practically identical to Shell No. 4 prior to the burst test. The mechanical properties of these tensile specimens are given in the following table:

specimens.

(2) Pre-stressing increases the yield pressure of a composite cylindrical vessel.

(3) A cylindrical vessel which is pre-stressed by an initial pressurization beyond the yield point will have a higher yield pressure due to the effect of the pre-stress and due to the effect of strain-hardening.

(4) Due to the increase in yield pressure a cylinder which has been pre-stressed by the aforementioned methods will have a better fatigue life when cycled than an equivalent cylinder of equal weight and volume which has not been pre-stressed.

While the present invention has been described in connection with fiber glass wound chrome-moly steel shells, it has been established that shells formed of other metals or metallic alloys and wound with other continuous filaments can be similarly benefited by the instant pre-stressing method. For example, the shells may be formed of stainless steel, aluminum alloys, titanium alloys or other non-ferrous alloys; and the filaments may be formed of ultra-high strength music wire or stainless steel wire. The materials, the number of turns and the winding tension preferably are selected to produce a pressure vessel having a high strength-to-weight ratio.

From the foregoing description, it will be seen that the present invention provides a simple, practical and economical method for producing pressure vessels having a; high strength-to-weight ratio.

It will be understood that the details and examples hereinbefore set forth are illustrative only and that the invention as broadly described and claimed is in no way limited thereby.

I claim:

1. The method of fabricating vessels for storing fluid Unde pressure Which method comprises applying a hoop winding under suflicient tension a plurality of layers of resin coated high tensile strength continuous filaments on a cylindrical wall section of a shell formed of high sterngth, ductile metal to establish a preceptable prestress in the cylindrical wall section, the shell having end walls one of which is provided with an opening for introducing and discharging fluid under pressure; curing the resin to provide a permanent filament resin laminate on the cylindrical Wall section; and thereafter subjecting the interior of the shell to fluid pressure at a value and for a sufficient duration of time to expand the shell and cause the yield strength to approach the ultimate tensile strength of the metal providing the cylindrical section to thereby establish a further perceptable pre-stress in the cylindrical wall section.

2. The method according to claim 1, wherein the tension at which the layers of resin coated filaments are Wound decreases substantially from the first layer to the last layer.

3. The method according to claim 1, wherein the fluid pressure to which the interior of the shell is subjected is at a value between the test pressure and the burst pressure of the vessel.

References Cited by the Examiner UNITED STATES PATENTS 1,232,110 7/1917 Sloper 156165 2,160,371 5/1939 Schnabel 156165 2,376,831 5/ 1945 Stearns 220-3 2,381,396 8/1945 Kuhn 2203 2,988,240 6/ 1961 Hardesty 2203 FOREIGN PATENTS 703,811 2/ 1954 Great Britain.

EARL M. BERGERT, Primary Examiner.

J. N. DRUMMOND, Examiner. 

1. THE METHOD OF FABRICATING VESSELS FOR STORING FLUID UNDER PRESSURE WHICH METHOD COMPRISES APPLYING A HOOP WINDING UNDER SUFFICIENT TENSION A PLURALITY OF LAYERS OF RESIN COATED HIGH TENSILE STRENGTH CONTINUOUS FILAMENTS ON A CYLINDRICAL WALL SECTION OF A SHELL FORMED OF HIGH STRENGTH, DUCTILE METAL TO ESTABLISH A PRECEPTABLE PRESTRESS IN THE CYLINDRICAL WALL SECTION, THE SHELL HAVING END WALLS ONE OF WHICH IS PROVIDED WITH AN OPENING FOR INTRODUCTION AND DISCHARGING FLUID UNDER PRESURE; CURING THE RESIN TO PROVIDE A PERMANENT FILAMENT RESIN LAMINATE ON THE CYLINDRICAL WALL SECTION; AND THEREAFTER SUBJECTING THE INTERIOR OF THE SHELL TO FLUID PRESSURE AT A VALUE AND FOR A SUFFICIENT DURATION OF TIME TO EXPAND THE SHELL AND CAUSE THE YIELD STRENGTH TO APPROACH THE ULTIMATE TENSILE STRENGTH OF THE METAL PROVIDING THE CYLINDRICAL SECTION TO THEREBY ESTABLISH A FURTHER PRECEPTABLE PRE-STRESS IN THE CYLINDRICAL WALL SECTION. 